When Phoenix residents felt the strong earthquake that rocked Baja California on Sunday, disbelief and a sense of excitement quickly spread.
"Earthquake?? Phoenix??" one resident wrote on Twitter. Another tweeted: "Wow, we actually DID feel the earthquake! We just didn't know what it was here in Phoenix!"
Longtime dwellers of this desert city — the fifth most populous in the nation — have grown accustomed to a sense of immunity from natural disasters. Now they find themselves wondering about such threats, considering three quakes have been felt in the area in roughly the last year.
Scientists say maybe so, maybe no.
Morgan Page, a Pasadena, Calif., geophysicist with the U.S. Geological Survey, said it's unlikely one as big as Sunday's 7.2-magnitude quake will occur again soon, but Phoenix could feel aftershocks in the next month.
She said the activity in the last year or so could be a cluster and a sign of things to come, or it could just be chance.
"There's no way to know for sure," she said.
Page said Phoenix could have felt the just two other earthquakes of 7.0-magnitude or higher in the past 35 years, one in 1992 and another in 1999.
Sunday's temblor occurred on a fault that hadn't ruptured since 1892. Erik Pounders, a USGS geologist, said "it's such a chaotic system" of faults that needs more research.
The quake struck just south of the U.S. border near Mexicali, killing two people and destroying dozens of businesses and homes there and severely injuring one person in the neighboring California town of El Centro. The shaking was felt hundreds of miles away in Las Vegas and Phoenix.
Residents saw their chandeliers and overhead lights swaying, some felt the buildings they were in move, and water splashed over the sides of swimming pools.
"Old timers here in Arizona say 'I've never felt anything like this before' and now suddenly they're feeling earthquakes," said Ed Garnero, a seismology professor at Arizona State University. "They think the world is coming to an end."
But it's just part of the Earth's natural system, said Matt Fouch, also an ASU seismology professor.
"We'll always feel them if they're in the general area of northern Mexico, Baja California and California and they're of a significant magnitude," Fouch said.
He said Phoenix has a unique risk in the event of a major quake in southern California or northern Mexico because it's in a large valley of sediment, as opposed to solid rock.
"All of Phoenix is sitting in a bowl of Jell-O, so if you excite the right frequency, then depending on the building height, some of those buildings can start to vibrate like a guitar string," he said. "Fortunately for Phoenix, we're not that close to these very large fault systems, so we wouldn't experience a very large event nearby, but we're still exposed."
Phoenix does not lie on any active fault lines, although parts of northern Arizona do.
USGS data shows that in the past 100 years or so, 14 temblors of moderate intensity have centered in Arizona. No earthquake in recorded history has caused deaths or injuries in the state.
During a two-week period in 1910, a series of 52 earthquakes scared away a construction crew in the Coconino National Forest after boulders rolled into their camp, and an August 1912 tremor caused a 50-mile crack in the earth north of Flagstaff, damaged homes in Williams and triggered rock slides that witnesses said made the earth appear to roll "like waves on the Colorado River," according to the USGS.